Interrelationships between plant functional types and soil moisture heterogeneity for semiarid landscapes within the grassland/forest continuum: a unified conceptual model

In semiarid landscapes, the ratio of herbaceous to woody plant biomass is a major determinant of ecosystem properties. This ratio depends to a large e xtent on the amount and spatial distribution of soil moisture that is avail able to plants, and these variables, in turn, are determined primarily by c limate and land use. Current conceptual models for determining the ratio of herbaceous to woody plant biomass in semiarid plant communities are based either on differences in soil moisture with depth (vertical heterogeneity) from one site to another (Walter's two-layer model) or on differences in so il moisture between canopy and intercanopy patches at the same site (horizo ntal heterogeneity) that result from disturbances associated with land use (Schlesinger et al.'s model of desertification). We developed a model that unifies these two perspectives by relaxing two assumptions of Walter's two- layer model. First, our model recognizes that soil moisture varies horizont ally between canopy and intercanopy patches, not only due to land-use distu rbance, a general assumption of the Schlesinger et al. model, but also due to the physical nature of the canopy itself. Second, while retaining the ge neral assumption of Walter that woody plants obtain moisture from deeper so il layers than do herbaceous plants, our model recognizes the existence of two types of woody plants: those that extract a substantial proportion of t heir moisture from deeper layers and those that extract mainly from shallow er layers. By modifying the two-layer hypothesis to include four soil compa rtments and distinguishing between shallow- and deeper-rooted woody species , our model integrates three key concepts in semiarid ecology: (1) the prop ortion of woody cover increases as moisture in the deeper soil layers incre ases (Walter's two-layer hypothesis for coexistence of herbaceous and woody plants); (2) land use practices that cause a reduction in herbaceous veget ation and compaction of intercanopy soils lead to a long-term increase in t he proportion of woody plants (Schlesinger et al.'s concept, or more genera lly, that at a given site multiple variations in the proportions of herbace ous and woody plant biomass are possible); and (3) changes in the ratios of herbaceous to woody plant biomass exhibit complex behavior (changes can ha ppen quickly and are not directly reversible without intensive management). This integration of concepts results because rather than assuming a simple , one-way dependence of plant functional types on soil moisture heterogenei ty, our model assumes an interdependence between the two: soil moisture het erogeneity constrains the composition of the plant community, which in turn modifies soil moisture heterogeneity. The four-compartment model that we p ropose enables, for the first time, an integrated picture of both dimension s of soil moisture heterogeneity - horizontal and vertical - and of the int erdependence between soil moisture heterogeneity and the proportions of the plant functional types that make up a given plant community. This unified conceptual model can be applied to provide insight into the individual and the combined effects of climate and land use on semiarid plant communities within the grassland/forest continuum, which vary in the proportions of can opy and intercanopy patches.